Among various performances which are required of internal combustion engines for automobiles, an emission performance is one of the performances especially considered to be important today. It is a catalyst which is disposed in an exhaust passage, and more particularly, temperature control of the catalyst that influences the emission performance. Purifying performance of the catalyst significantly depends on its temperature. Therefore, various inventions relating to temperature control of a catalyst have been conventionally made. As the literatures that disclose such inventions, for example, Japanese Patent Laid-Open No. 2005-171919, Japanese Patent Laid-Open No. 2008-106713, Japanese patent Laid-Open No. 2007-162634 and Japanese patent Laid-Open No. 2003-195913 can be cited.
Incidentally, the temperature in the catalyst during operation of the internal combustion engine is not always uniform. FIGS. 6A, 6B and 6C show an example of a temperature distribution which occurs in the catalyst. FIG. 6B shows the temperature distribution in a I-I direction, that is, a flow path direction of the catalyst in a schematic diagram of FIG. 6A in graphical form. FIG. 6C shows the temperature distribution in a II-II direction, that is, a diameter direction of the catalyst in the schematic diagram of FIG. 6A in graphical form. Directly after start of the internal combustion engine, a carrier itself of the catalyst is cooled, and therefore, the temperature distribution in the flow path direction becomes especially remarkable. Therefore, the catalyst temperature sometimes does not rise so much at an outlet port side of the catalyst even though the catalyst temperature rises to a temperature at which a sufficient reaction rate for purifying emission can be obtained at an inlet port side of the catalyst. In reality, a temperature difference between the inlet port side and the outlet port side of the catalyst sometimes reaches 500° C. or more depending on operation conditions of the internal combustion engine.
It has been a well-known fact for a long time that the temperature distribution as above occurs in the catalyst. Therefore, the conventional inventions relating to temperature control of catalysts include the inventions which perform temperature control in consideration of the temperature distributions in catalysts. For example, in the invention disclosed in Japanese Patent Laid-Open No. 2005-171919, the temperature difference between a front end and a rear end of a catalyst is obtained, and a flow rate of secondary air which is supplied to an exhaust system is regulated so that the temperature difference has a value within a predetermined range.
However, many inventions which have been conventionally proposed are made from the viewpoint of how the temperature distribution in the catalyst is reduced. In enhancing the emission performance of the internal combustion engine, it is important how efficiently the entire catalyst is used, but the invention which deals with the temperature distribution in the catalyst from the viewpoint like this is not found at present.